The following U.S. Patents and Applications will be discussed:
2005/0094833 (appln.) to Bloodworth et al.;
2002/0122561 (appln.) to Pelrine et al.;
2002/0076069 (appln.) to Norris et al.;
U.S. Pat. No. 6,760,455 to Croft, III et al.;
U.S. Pat. No. 6,535,612 to Croft, III et al.;
U.S. Pat. No. 6,502,662 to Nakamura et al.;
U.S. Pat. No. 6,393,129 to Conrad et al.;
U.S. Pat. No. 6,304,662 to Norris et al.;
U.S. Pat. No. 6,188,772 to Norris et al.;
U.S. Pat. No. 3,778,562 to Wright;
U.S. Pat. No. 3,668,335 to Beveridge;
U.S. Pat. No. 3,345,469 to Rod;
U.S. Pat. Nos. 3,008,014 & 3,008,013 to Williamson
U.S. Pat. No. 2,975,243 to Katella;
U.S. Pat. No. 2,615,994 to Lindenburg et al.;
U.S. Pat. No. 1,930,518 to High;
GB Patents:
537,931 Jul. 14, 1941 to Shorter Referring to the above-listed patent documents:
Bloodworth et al. teach electrostatic loudspeaker stator panels made using a fiber-glass printed circuit board process. It discusses the difficulty of insulating punched perforated metal stator panels due to their intrinsically sharp corners and presents the use of PCB material with centrally encapsulated conductors as an alternate means of manufacturing high-performance stator panels. This patent is of interest in relation to the problem of obtaining adequate insulation on a stator panel for use at high voltages.
Pelrine et al. teach a multilayer polymer film structure that utilizes a film that is supported at close intervals and requires a bias pressure to predispose the film into small, part-spherical radiating bubble elements. Such a transducer would allow limited membrane excursion and be suited only for higher frequency sound reproduction. Reference is made to the film being deformable into different shapes such as cylindrical or spherical; however such a polymer film would require an elaborate support structure.
The Norris et al. application utilizes a sonic emitter with a foam stator having a conductive acoustic film in proximity on one side and a sparse conductive coating on the other side. A high voltage bias is then applied to the two surfaces of the foam structure causing the acoustic film to move towards the foam due to electrostatic attraction. Reference is made to the foam structure being deformable into a cylinder or even a spherical shape, although no embodiment is shown of the spherical case.
In U.S. Pat. No. 6,760,455, Croft et al. teach the use of a distributed filter within a planar electrostatic loudspeaker to decrease the effective radiating area with increasing frequency in order to maintain angular dispersion of high frequency sound waves. Croft suggests that this technique can be used to simulate an ideal spherical point source radiator. The active radiating area would have to become very small at the highest audible frequencies in order to maintain modest dispersion thereby limiting the effective radiating power at higher frequencies due to the small effective area in use.
In U.S. Pat. No. 6,535,612, Croft et al. refer to a structure and method for applying tension to the acoustic diaphragm without relying on edge tensioning. It teaches structures that provide mechanical biasing by predisposing the film into a corrugated shape. Described is a corrugated planar panel and cylindrical one-axis curved shape for improved dispersion. Croft states that “two cylindrical corrugated stators 356 create a hemispherical shape and a non-planar diaphragm 360 is arranged between the two opposing stators”. The shape is similar in form to that of a beehive with sound being radiated by a discontinuous corrugated diaphragm. The Croft structure, while claiming a “diaphragm securing structure and method”, would limit available diaphragm excursion and hence low frequency reproduction capability.
The Nakamura et al. speaker, although a piezo electric transducer, is of interest due to its hemispherical shape wherein the structure grows and contracts radially outward. Such a transducer would only be capable of producing higher frequencies due to limited deformation capability.
Conrad et al. depict a paper based electrostatic transducer. The form of the structure is corrugated similarly to that of Croft et al., and it also shows the identical beehive depiction as a hemispheric radiator.
In the U.S. Pat. Nos. 6,304,662 and 6,188,772 patents, Norris begins his discussion of electrostatic speakers fabricated with foam stators, which are further described in the application listed above.
Wright teaches an electrostatic loudspeaker having an acoustic wave-front modifying device and resultant polar radiation pattern. The art depicts a number of progressively angled flat facets arranged so as to provide dispersion of sound in both the horizontal and vertical planes. Furthermore, the loudspeaker is encapsulated in a dense gas that is said to provide a desired acoustic wavefront shaping and increased dielectric breakdown capability for improved power handling.
Beveridge teaches a sophisticated mechanical lensing structure to transform a planar wave front from a flat electrostatic radiator into a cylindrical wavefront with dispersion about a single axis.
Rod teaches a bendable electrostatic sheet transducer comprised of outer wire mesh stators and a centrally located electrically conductive acoustic membrane located adjacent to insulating dielectric layers. The transducer is shown in various forms including flat and cylindrical. It is also shown that the bendable sheet could be formed into a substantially continuous 360 degree surface of revolution, of cylindrical or frusto-conical form, and used to construct the shade of a household lamp having contained in its base a conventional electromagnetic loudspeaker for lower frequency reproduction. With the lamp depicted by Rod a listener would be required to maintain their ears within the projected height of the lamp shade in order to hear higher frequencies. The subject of the present application includes an embodiment of a lamp, but with improved vertical dispersion of sound waves.
Williamson, in the '013 patent, teaches a method of using a series of planar electrostatic panels and progressive delay lines so as to generate a tilted non-parallel wavefront. The patent also teaches a method of improving dispersion of high frequencies by using a second step-up transformer to drive a smaller annular section of a larger circular planar diaphragm. The '014 patent teaches similar planar panels in a zigzag configuration.
Katella teaches an electrostatic loudspeaker with an improved membrane support method. The insulating spacer panels have cut-outs that are oriented in an oblique or spiral arrangement in order to provide improved mechanical and acoustic properties as compared to square or co-axial cut-outs. According to Katella: “it is known to be important that the plates {stator panels} be definitely curved about some suitable axis or axes”. No additional reference is made to the term “axes” such as in relation to modifying the dispersion characteristics of a transducer in a second vertical direction in addition to the disclosed cylindrical form, and as such the meaning of the term “axes” as compared to “axis” is thus limited to a preferred shape as would be required so as to cause the membrane to contact said spiral cut-outs. According to Katella, the stator plates 13 and 14 are formed of un-insulated metal and the vibrating membrane itself has an insulating layer on either side of a conductive core. A thick insulating layer would be required adjacent to said conductive core to enable high voltage operation and as such the insulated membrane would exhibit a reduction in high frequency reproduction capability due to increased mass.
Lindenburg teaches a diaphragm for electrostatic loudspeakers consisting of five layers having outer foil layers adjacent to inner compressible paper layers with a center insulating spacer. The structure traps a small volume of compressible air and as such, could radiate sound. The diaphragm would however be limited to very high frequency reproduction due to the limited compressibility of the thin film of trapped air. Also of interest is a figure that depicts a formed foil and paper structure that is curved about both longitudinal and vertical axis for improved dispersion of sound at higher frequencies.
In U.S. Pat. No. 1,930,518 High taught an electrostatic loudspeaker panel with a mechanism for controlling the tension and position of the diaphragm. The linear dielectric support structure used had many laterally spaced supports thus creating discrete facets that were to be tensioned using mainly the force of the electric field. In practice however, if the diaphragm were slack as it were, without sufficient tension, it could still flap back and forth between stable positions and hence affect sound reproduction. As the structure provides a series of long lineal facets it is also shown in the form of an approximated arc, as is a common present day practice for electrostatic loudspeaker panels. The patent also suggests that the structure could be used to approximate a spherical shape if the width of the facets were modified to form lunes of a sphere, although no embodiment is shown. Although the High disclosure dates from 1933 it appears that there has been no commercial use of electrostatic loudspeaker having a structure that is curved about two axes. High uses stator members of semi-conductive material, such as artificially prepared slate.
The technology in the GB 537,931 patent and many related patents form a core technology that is still in use today in designs of commercial electrostatic loudspeakers offered by the Quad Hi-fi company of the UK. These designs center around the use of a large planar diaphragm utilizing a novel stator that is subdivided into electrically isolated concentric annular regions. The audio signal applied to the annular stator regions is then progressively modified so as to cause the flat panel to emit an approximated spherical wave-front. What is of significant note is that Quad holds the claim of marketing the only full-range point source electrostatic loudspeaker and has held to this claim for about 60 years.
Although not an electrostatic loudspeaker, the Radialstrahler loudspeaker system manufactured by MBL of Germany is of interest as it provides a continuous 360-degree horizontal dispersion of sound. According to the manufacturer “The Radialstrahler concept includes a circular vertical arrangement of lamellas around an axis for each frequency range (tweeter, midrange driver and subwoofer)” A frequency range of approximately 100 to 20,000 Hertz can then be reproduced with an unique 3-way system of cooperative “football” shaped acoustic transducers, each of decreasing size for increasing operating frequency. The groups of vertically arranged curved lamellas are actuated at their respective ends in the vertical direction with electromagnetic voice coil drivers, thereby expanding radially outward and inward.
In actuality, there are several commercial ESL systems on the market utilizing flat panel radiators as well as cylindrical panels curved about a vertical axis. One company of note that offers a complete line of loudspeaker systems utilizing “line source” cylindrical ESL panels is that of Martin Logan. All of these systems are comparable in stature to that of an adult human.